1. Field of the Invention
This invention relates to electrostatic recording media of the type having a double-layer coating formed on a support for use in facsimile or high speed printing.
2. Description of the Prior Art
Typical of known electrostatic recording media are those which comprise a conductive layer which is formed on one surface of a support and has a surface resistivity of 10.sup.5 -10.sup.11 .OMEGA. and a dielectric layer coated on the conductive layer and made of a dielectric material whose specific resistance is as high as over 10.sup.12 .OMEGA..multidot. cm. Ordinarily employed conductive layers are formed by impregnating electrolytes such as lithium chloride in slick or wood free papers or by coating cationic polyelectrolytes such as high molecular weight quaternary ammonium salts on supports. However, these conductive layers utilizing ion conductivity have drawbacks in that their surface resistivity is greatly influenced by ambient humidity and sharply increases with the relative humidity is below 20%, making it almost impossible to record.
In order to overcome this drawback, there has been proposed a method in which a conductive material having electron conductivity such as cuprous iodide or silver iodide is used as a conductive layer. That is, it is intended to make the recording at low humidity by using the electron-conductive material without undergoing any influence of ambient humidity. However, these materials undesirably assume color and have such an unfavorable property that since the electron conductivity results from an excess of iodine, the iodine is released when an electrostatic latent image is developed and then fixed by application of heat.
Various improvements have been proposed including methods in which conductive zinc oxide is used instead of the conductive metal halide materials and is applied onto a support by use of hydrophobic binders to give a conductive layer (Japanese Laid-Open Application Nos. 51-25140 and 54-126029) and a method using copper iodide and zinc oxide in combination as a conductive layer (Japanese Laid-Open Application No. 54-126835).
However, all of these improved methods have the drawback that the recording density is lowered when the ambient humidity exceeds about 75% R.H. Presumably, this is because the conductive particles in the conductive layer deteriorate in electric contact when a support or base grows by absorption of moisture, thus increasing the surface resistivity.
In order to further improve these methods, we have already proposed in our copending U.S. application No. 054,901, filed July 5, 1979, now U.S. Pat. No. 4,275,103, an electrostatic recording medium which makes use of a conductive layer made of a conductive metal oxide semiconductor and an organic binder including a polyelectrolyte. This medium has an excellent dissolving power and an excellent moisture-resistant characteristic. The reason why the moisture-resistant characteristic is excellent is considered to be as follows: The ion conductivity of the polyelectrolyte contributes to maintenance of the surface resistivity even under high humidity conditions where the electric contact of the conductive particles tends to deteriorate. In this connection, however, zinc oxide which is advantageous from the viewpoint of economy and whiteness over other metal oxide semiconductors, still has a problem that the surface resistivity of the zinc oxide conductive layer becomes unstable as time goes.